The use of solar panels in spacecrafts such as satellites is widely known. These equipment once placed in operation position, for example a satellite being placed in the orbit, are usually subject to effects caused by the space environment in which they are positioned. Among the various effects in the space environment, an important one is the surface electrical charging which is caused by the accumulation of electric charge on the surface of the spacecraft. One of the sources for causing such surface electrical charging is the solar radiation.
Solar panels used on spacecrafts are subject to solar radiation and therefore are very likely to suffer from accumulation of electric charge on their surface. The surface of a solar panel is usually covered with a glass material, the so called “cover glass”.
The cover glass of solar panels are therefore charged with such electric charge where this charge is positive with respect to the mass (ground) of the spacecraft. As it is known, the positive charge is accumulated due to the effect of a strong photoemission current caused by the action of expelling of electrons (caused by solar radiation). Another source of positive charge is the secondary electron emission (SEE) or extraction of electrons due to the impact of energetic electrons from the space environment. In practice, typical materials used for cover glass and surface treatments give rise to a high SEE yield.
The cover glass material is also highly resistive, hence the surface of the cover glass is electrically isolated from the mass of the spacecraft.
On the other hand, the rear face of the solar panel, the “dark” side, which is electrically connected to the mass of the spacecraft, is charged negatively. For example in the case of a satellite the rear face of the solar panel is usually made of a conductive material with a very low emissive property of the negative electric charge. As a result, energetic electrons from the plasma are captured by the conductive part giving rise to an accumulation of negative charge.
In consequence, a relatively large accumulation of the positive charge on the front surface with respect to the negative charge on the mass can cause electrostatic discharge between the surface and the mass which can damage seriously the circuitry in the spacecraft. In practice, about 500 V is generally considered as a typical threshold value for producing a discharge. In order to avoid such undesired electrostatic discharge certain solutions are already known. One of such known solutions is to cover the cover glass with a conductive coating which is then connected to the “earth” of the spacecraft. However the costs involved in this solution are very high. Furthermore, the lifetime of the conductor coating is relatively limited, which makes this solution not advisable for spacecraft with a long lifetime.
Another known solution for the above problem is that of using a neutralizer. This is done by using a gun which is charged with neutralizing material such as electrons, cold positive ions or cold plasma (plasma being an ionized gas having equal amounts of positive and negative charges). The gun “shoots” a specific amount of electrons, ions or plasma in the vicinity of the spacecraft in order to neutralize the charge accumulated on the surface of the spacecraft. However this solution suffers from a high cost involved in its implementation and operation, and its low effectiveness, especially in the case where electrons are used.
A further solution for avoiding electrostatic discharge is known from U.S. Pat. No. 6,243,243. According to the solution disclosed in this document the spacecraft ground terminal and the solar panel array are coupled together through a directional high impedance device that increases the impedance in the direction from the solar panel array to ground and in this manner prevents any potential differences on the spacecraft from completing an electrical path into the ground terminal thus preventing electrostatic discharge between the solar panel array and the ground terminal. However this solution is more directed toward reducing the impact of the potential difference caused by electric charge accumulation rather than avoiding it because the potential difference, according to this solution, is not prevented from being built-up.
It is therefore desired to provide a solution for preventing electrostatic discharge without the above drawbacks.